Two-tube shock absorbers are known, as for example from my earlier patent 3,749,210 as well as from U.S. Pat. Nos. 2,943,711, 3,661,236, and 4,005,769 and German utility model No. 7,619,540. A standard such shock absorber has an outer tube centered on an upright axis and forming with an inner tube an outer gas/liquid chamber. A piston rod inside the inner tube forms therewith an inner liquid chamber that can be pressurized by axial displacement of this piston rod and of the piston attached to it that rides on the inner wall of the inner tube. A foot valve allows limited flow between the inner and outer chambers. The inner chamber is completely filled with liquid, and the outer chamber is partially filled with this liquid and has a pressurized head of air above the liquid.
Normally the piston rod passes out of the absorber through a guide-seal assembly that incorporates at least one hard synthetic-resin guide bushing and inner and outer soft seal rings that axially flank this bushing. The inner seal protects the outer seal against the high pressure reigning in the inner chamber as the piston moves axially up. The guide-seal assembly is formed with a passage communicating between a compartment formed between the inner and outer seals and the outer chamber. A check valve in the guide-seal assembly permits flow from the inner chamber to the outer chamber for bleeding air from the inner chamber.
The inner seal in an arrangement such as described in above-cited German utility model No. 7,619,540 is spaced axially down from the guide bushing, normally being provided in a further inner tube extending down from the guide-seal assembly. This inner seal normally lies below the liquid level in the outer chamber. As some lateral bending of the piston rod is common, this inner seal and the elements carrying it must normally be built to permit limited radial shifting. Such construction not only leads to premature wear for this inner seal, which must be able to resist high pressure, but also makes lubrication of the guide bushing and outer seal difficult.